Origin and history of sluggish

late 14c. (late 13c. as a surname), slogard, "habitually lazy person, one afflicted with the sin of sloth," with -ard + Middle English sluggi "sluggish, indolent" (early 13c.), which is probably from Scandinavian; compare dialectal Norwegian slugga "be sluggish," sluggje "heavy, slow person," dialectal Swedish slogga "to be slow or sluggish."

'Tis the voice of a sluggard — I heard him complain:

"You have wak'd me too soon, I must slumber again."

[Isaac Watts 1674-1748]

***

'Tis the voice of the Lobster: I heard him declare

"You have baked me too brown, I must sugar my hair."

["Lewis Carroll" (Charles L. Dodgson), 1832-1898]

 As an adjective meaning "sluggish, lazy" from 1590s. Related: Sluggardly; sluggardize. Sluggardie "idleness, indolence" is from late 14c.; sluggardry is by 1510s.

adjectival word-forming element, Old English -isc "of the nativity or country of," in later use "of the nature or character of," from Proto-Germanic suffix *-iska- (cognates: Old Saxon -isk, Old Frisian -sk, Old Norse -iskr, Swedish and Danish -sk, Dutch -sch, Old High German -isc, German -isch, Gothic -isks), cognate with Greek diminutive suffix -iskos. In its oldest forms with altered stem vowel (French, Welsh). The Germanic suffix was borrowed into Italian and Spanish (-esco) and French (-esque). Colloquially attached to hours to denote approximation, 1916.

The -ish in verbs (abolish, establish, finish, punish, etc.) is a mere terminal relic from the Old French present participle.